Abstract: In this paper, based on the laser mirror welding process of 2219 aluminum alloy, the thermo-flow coupling model of laser mirror welding process was established, and the dynamic characteristics of keyhole were quantitatively solved and analyzed. It is found that the keyhole is coupled rapidly and then maintained small fluctuations during the laser mirror welding process. Before keyhole coupling, the cross-section area difference of keyhole on both sides is small, and the maximum difference is only 0.35 mm²; After keyhole coupling, the cross-sectional area of keyhole is kept increasing and fluctuated within a certain range at 120 ms. The increase of heat input can promote the coupling degree and stability of keyhole. At the same time, the laser power has a greater influence on the coupling area of keyhole, while the welding speed has a greater influence on the area of keyhole opening. Based on the above rules, the optimized welding process window for laser mirror welding of 2219 aluminum alloy with 6 mm thick is summarized.